HEXAPODA. 63 



the other segments, and it may also differ greatly in form 

 from them ; under such circumstances it is sometimes desig- 

 nated the metatarsus (met-a-tar'sus) (Fig. "jy^ m). The last 

 segment of the tarsus usually bears one or two claws. 



On the ventral surface of the segments of the tarsus in 

 many insects are cushions of short hairs or of membrane, 

 capable of inflation, or concave plates, which act so as to 

 produce a vacuum, and thus enable the insect to walk on the 

 lower surface of objects. These cushions or plates are called 

 piilvilli (pul-virii). In many insects the pulvillus of the last 

 segment of the tarsus is a circular pad projecting between 

 the tarsal claws. In most descriptive works this is referred 

 to as the pulvillus, even though the other pulvilli are well 

 developed. See also page 420. 



The Wings, — The two pairs of wings are borne by the 

 mesothorax and metathorax, but either or both pairs may 

 be wanting. Thus the Flies, or Diptera^ have only the first 

 pair of wings fitted for flight, the second pair being repre- 

 sented by a pair of knobbed threads, the function of which 

 is unknown ; and with the Earwigs and Beetles the first 

 pair of wings is wanting, although the mesothorax bears a 

 pair of horny wing covers, which are somewhat wing-like in 

 form, and are commonly described as wings.^ 



In form an insect*s wing is a large membranous append- 

 age, which is thickened along certain lines. Tliese thickened 

 lines are termed the veins or nerves of the wing; and their 

 arrangement is described as the venation or neuration of the 

 wings. The thin spaces of the wings which are bounded by 

 the veins are called cells. When a cell is completely sur- 

 rounded by veins it is said to be closed ; but when it extends 

 to the margin of the wing it is said to be open. 



The wings of different insects vary greatly in structure, 



* The wing covers or elytra of earwigs and beetles probably correspond to 

 the tegulae of Hymenoptera and to the patagia of Lepidoptera; that is, they 

 are a pair of the side pieces of the mesothorax, the paraptera, greatly en- 

 larged. 



